As I work in a lingerie shop we see some funny things.. just looking around the internet the other day, this site popped up that told you how to measure yourself for a bra.
First of all it said to measure underneath your bust which is right, but then they said add on 4 to an even number and add on 5 to an odd number. Why they do this I have no idea.. if you measure 32 you are a 32 back? A good guide is:
Size 6-8 28-30 back
Size 8-10 30-32 back
Although, my mums a size 14 and she wears a 32. Obviously the tighter your band the more support you get which relives things like straps falling off, saggyness, stretch marks etc and the main point being your clothes fit much better.
Apparently the average size is something ridiculous like a 34B? I would say its more an F cup. There really isn't such a thing as a 34B.. it would mean a lady in a size 14 with no boobs. Young girls who wear 34Bs are in the wrong size and probably need a 30D?
So, go get measured properly. And, by properly I mean, go to good fitters. Not m&s or la senza!!!